Originally Posted by Loy58
differentiated classes within a grade
In theory this may sound good, however... and unfortunately...
1) Many schools are reluctant to have high students in a separate classroom than low students because they believe it is unfair to the teacher.
2) Even when classes are somewhat grouped by ability and readiness, there can be a wide range of skills, talents, and/or gifts among students.
3) A gifted kid may be well above even the next brightest kid(s) in their grade level and may benefit from cluster grouping by readiness and ability in each subject, regardless of chronological age or grade level (multi-age classroom). These links provide more information on flexible cluster grouping by readiness and ability:
http://www.casenex.com/casenet/pages/virtualLibrary/gridlock/groupmyths.html,
http://www.gifted.uconn.edu/nrcgt/reports/rbdm9204/rbdm9204.pdf
4) While many "gifted programs" may consist of teaching a subject one grade level advanced, this may not be enough for a gifted student.
5) Research studies reveal that gifted kids are in such small numbers that classrooms are padded with non-gifted high achieving kids, which may then outnumber the gifted kids. In a typical classroom of 20-24, the teaching/instruction may then be targeted to this majority population and be well below the zone of proximal development (ZPD) of the gifted kid(s).

Having said that, I agree with Loy58 in that having a somewhat narrower range of student abilities within a classroom, it may become more realistic for a teacher to provide meaningful leveled instruction for groups of students in that classroom.

Many private schools pride themselves in having all gifted and/or high achieving kids, therefore they may believe that they are already providing this type of high ability level classroom experience to their enrolled pupils.